Activitity Parameter and Return Value Not Retaining State

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1

Hello
I have a simple workflow with Couple of activities
First Activity is fetching data from downsteam system using REST API
then it adding few more addition properties
and then return that value

@Data
class DownstreamFirstActivityMethodResponse implements Serializable {
//serial version id included here

private String prop1;
private String prop2;
/**/
private String firstActivityMethodSettingAsAdditionalParam;

}

class FirstActivityParam implements Serializable {
//serial version id included here
private String workflowIsSetting;
private String firstActivityMethodSettingAsReturnParam;
}

Signature of the first activity:

DownstreamFirstActivityMethodResponse firstActivityMethod(FirstActivityParam param)

Note: firstActivityMethod does 3 things
1) param.setFirstActivityMethodSettingValueAsReturnParam("first activity setting value")
2) Calls REST API and result of downstream is an instance of DownstreamFirstActivityMethodResponse
3) sets a new prop - 
downstreamFirstActivityMethodResponse.setFirstActivityMethodSettingAsAdditionalParam("THIS IS NEW PROP VALUE SET BY 1st ACTIVYT");

Signature of the second activity:

void secondActivityMethod(FirstActivityParam param, DownstreamFirstActivityMethodResponse input )



workflow logic - inside workflow method:
--------------------------------------

FirstActivityParam param = new FirstActivityParam();
     param.setWorkflowIsSetting("somevalue");

DownstreamFirstActivityMethodResponse firstActityReturnValue = firstactivity.firstActivityMethod(param);

System.out.println(firstActityReturnValue.getFirstActivityMethodSettingAsReturnParam());
      //Expected : THIS IS NEW PROP VALUE SET BY 1st ACTIVYT
      //BUT - is NULL

System.out.println(FirstActivityParam.getFirstActivityMethodSettingValue());
      //Expected : "first activity setting value"
      //BUT - is NULL
      
       
secondActivityObject.secondActivityMethod(param,firstActityReturnValue)
//2nd Activity Needs those additional return param value and addional prop value from downstreamResponse

Question : it s really a puzzle for me to see that, after 1stActivityMethod call completion - those addional elements are nullfied, why